The present invention relates to a novel and efficient method for the manufacture of connecting rods. The invention is particularly advantageous for the manufacture of connecting rods for use in small internal combustion reciprocating engines. Such engines have low horsepower output and are utilized in machines which perform a wide variety of useful functions. For example, engines which would typically employ connecting rods manufactured according to the method of the present invention include weed trimmers, lawn edgers, handheld chain saws, and go-carts.
Historically, manufacturers of connecting rods for these small, low horsepower engines have utilized the same general method of manufacturing used by manufacturers of larger rods. Typically, the first step in such prior art method has been to forge the two parts which together make up a connecting rod, that is, the shank and the cap. This two part construction is necessary for the assembly of the connecting rod to the crank shaft pin. The next step involves grinding flat the upper and lower horizontal surfaces of the forging. Additional machining steps include the machining of the inner and outer configurations of the rod, the respective surfaces of the shank and the cap which are joined together, and any serrations in such surfaces or similar means for insuring non-slippage at the shank-cap juncture. Bolt holes for securing the cap to the shank are then drilled and tapped, followed by the machining of the crank pin and wrist pin bores. Following heat treatment of the rod, the inner diameters of the bores are ground to the proper tolerances.
The above-described method is far from satisfactory with respect to the manufacture of connecting rods for small engines. First, the process involves far too many steps so that the connecting rods cannot be economically produced fast enough to meet the great demand for them. Secondly, the reduced size of the connecting rods makes it very difficult to maintain dimensional accuracy throughout the several grinding and machining steps. Thus, the costly and slow-paced construction of connecting rods for small reciprocating engines has become a serious problem in an extremely competitive and cost-conscious industry.
In an attempt to solve the cost and production inefficiencies of the above-described prior art method of manufacture, several manufacturers of small connecting rods have modified the method slightly. Instead of forging the shank and cap separately, the modified method involves the forging of a single connecting rod configuration. The usual steps of grinding the outer surfaces, machining the inner and outer configurations, and drilling and tapping bolt holes follow. Then shallow slots are cut in the rod where the shank and cap portions normally meet, and following the heat treatment, the rod is broken at the slots to produce separate shank and cap portions.
The obvious advantage of this modified method is the elimination of the necessity of machining the mating surfaces of the shank and cap. Nevertheless, there are still many disadvantages associated with this method which precludes it from being a solution to the inefficiencies of prior methods. For example, it has been found that the rate of production of connecting rods utilizing this method is still too slow and uneconomical to meet the demand for such rods. Usually the rods are broken manually which is not only a costly operation, but time consuming as well.
Probably the most disadvantageous aspect of this breaking process is the extremely high scrap rate experienced, typically on the order of 30-40%. The caps broken off from the shank are often lost and the entire part has to therefore be discarded. Furthermore, if the part is hit too hard or inaccurately, it does not break correctly at the slot and again the part becomes scrap. Even if the part breaks at the slot, there is no assurance that the two pieces will mate when joined, as chips oftentimes are broken off and their absence prevents the mating of the shank to the cap.
There are further disadvantages in the heat treatment phase of this modified method. Contributing to the extremely high scrap rate is the warpage of the rods experienced during heat treatment. Warpage, or distortion as it is frequently referred to, during the heat treatment of metals is a function of the amount of growth of the crystalline units or "grains" which structurally constitute the metal being treated. The greater the growth of the grains during heat treatment, the greater the likelihood of warpage of the part. The two factors which contribute to large grain growth are the high temperature to which the metal is subjected, and the period of time of the heat treatment itself.
It is common for a low-carbon steel to be used in the production of small connecting rods. Therefore, it is advantageous to add carbon during the heat treatment process so as to provide for strength in the part. This carbon additive process is called "case hardening." Case hardening, however, lengthens the period of the heat treatment since time is required for the carbon atoms to diffuse into the grain structure of the metal. Furthermore, a hot worked metal, such as a forging, must be heat treated at a higher temperature than similar cold worked metals. These factors, taken together, explain the warpage suffered by the connecting rods manufactured under the breaking method. Such warpage also contributes, of course, to the high scrap rate of that method. Even if the part is not scrapped, a warped part must be machined before it can be functional. Thus, the manufacturer incurs added time and expense.
In order to avoid the problems of the breaking method, a few manufacturers have begun to produce single piece connecting rods. Although simplifying connecting rod manufacture, these single piece connecting rods create substantial problems with respect to the manufacture of complimentary crank shafts. Obviously, a single piece connecting rod requires either a two piece crank shaft, (and thus substantially increases the cost of manufacture and assembly of the crank shaft) or a single piece crank shaft with counter weight missing (a crank shaft generally suited to only the smallest of the small reciprocating engine class.)